1. Field of the Invention
The present invention relates to a process for preparing mixtures of diphenylmethane diisocyanates and polyphenylpolymethylene polyisocyanates known as raw MDI, having a reduced iodine color number and a reduced chlorine content by reacting the corresponding mixtures of diphenylmethanediamines and polyphenylpolymethylenepolyamines, known as raw MDA, with phosgene in the presence of at least one inert organic solvent, wherein a mixture of water and at least one monohydric or polyhydric polyoxyalkylene alcohol in an effective amount is incorporated into the reaction mixture after phosgenation is complete.
Raw MDI, one of the industrially most important starting materials for preparing polyisocyanate polyaddition products, for example urethane foams or foams containing urethane and isocyanurate groups, and for preparing diphenylmethane 4,4'-diisocyanate, an important formative component for producing polyurethane (PU) elastomers, or fibers, sealing compounds, adhesives, etc. is prepared in a known manner by reaction of raw MDA with phosgene, customarily in the presence of an inert organic solvent. Raw MDA is in turn obtained by condensation of aniline and formaldehyde in the presence of acid catalysts, with the percentages of diphenylmethanediamines and the homologous polyphenylpolymethylenepolyamines and their isomers being able to be controlled as a function of the mixing ratios selected for the starting materials and of the reaction conditions and the various processes (Kunststoff-Handbuch, Volume 7, Polyurethane, 1st Edition 1966 and 2nd Edition 1983, Carl-Hanser-Verlag, Munich, Vienna). If the condensation of aniline and formaldehyde is, for example, carried out in the presence of weakly acid catalysts, the raw MDA mixtures obtained have a relatively high proportion of 2,2'- and 2,4'-diaminodiphenylmethanes, while raw MDA mixtures having a high content of 4,4'-diaminodiphenylmethane and at the same time a low proportion of 2,4'-diaminodiphenylmethane can be prepared only in the presence of relatively large amounts of strongly acid catalysts, preferably strong mineral acids such as hydrochloric acid.
The ratio of the diaminodiphenylmethane isomers to the higher homologs in raw MDA is also dependent on the aniline/formaldehyde ratio and the condensation temperature, with larger aniline/formaldehyde ratios and low condensation temperatures giving high diaminodiphenylmethane contents (CA-A-770 026).
A disadvantage of these preparative processes which are described in many literature and patent publications is the formation of more or less strongly colored raw MDA whose color can vary from black through darker and paler brown shades to ochre. A further disadvantage is that these discolorations are not, or only insufficiently, reduced by the subsequent reaction with phosgene for preparing the corresponding raw MDI and the raw MDI formed cannot be purified by distillation. Furthermore, this undesired discoloration affects the downstream products, so that the polyisocyanate polyaddition products, which may be cell-containing, produced from colored raw MDI are also not colorless. Although the intrinsic color of the polyisocyanate polyaddition products does not adversely affect their mechanical properties, essentially colorless products are desired by the consumer.
There have therefore been many attempts to reduce the discoloration of raw MDI and to stabilize the polyisocyanates produced by means of suitable process measures or additives.
According to U.S. Pat. No. 2,885,420, organic polyisocyanates can be stabilized against discoloration by the addition of from 0.01 to 0.5% by weight, based on the weight of polyisocyanate, of an aromatic, cycloaliphatic or aliphatic ether or thioether.
To eliminate impurities which act as gel-formation catalysts in organic diisocyanate solutions, from about 0.001 to 1% by weight, based on the weight of the diisocyanate, of phosphoric acid is, according to DE-A-1 280 855 (GB 1 097 219), added to the solutions.
GB-B-1 465 014 describes the addition of glycidol in an amount of from 0.001 to 0.25% by weight, based on the weight of diisocyanate, for improving the storage stability of distilled diphenylmethane diisocyanates.
EP-B-0 183 976 (U.S. Pat. No. 4,677,221) relates to a process for preparing thermally color-stable (cyclo)aliphatic diisocyanates, in which process technical-grade diisocyanate is heated with aliphatically and/or cycloaliphatically bonded isocyanate groups in the presence of from 0.1 to 3% by weight of a compound soluble in the diisocyanate, which compound contains at least 3% by weight of structural units of the formula --NH--CO--, at from 100 to 220.degree. C. for a period of up to 5 hours and the diisocyanate thus treated is subsequently purified by distillation. The process cannot be applied to the treatment of raw MDI since, as already indicated, this is not distillable.
According to U.S. Pat. No. 4,465,639, raw MDI has incorporated into it, after phosgenation is complete but before complete removal of the phosgene, from 0.1 to 5% by weight of water, based on the polyisocyanate weight in the reaction mixture. The color of the raw MDI and the PU foams produced therefrom can be lightened by this measure. Furthermore, the proportion of higher molecular weight MDI homologs in the raw MDI is considerably lowered and their viscosity is reduced. Although the iodine color number of the raw MDI can be lowered in this manner, this method is also associated with considerable disadvantages. The presence of water considerably increases the corrosive action of the reaction mixture containing chlorine, hydrogen chloride and phosgene on the equipment of the production plant and thereby increases the risk of leakage, associated with an escape of toxic phosgene or a phosgene-containing reaction mixture. For safety reasons, prevailing opinion is therefore that moisture in any form should be essentially completely excluded in phosgenation.
According to EP-A-0 467 125, the iodine number of raw MDI can be reduced by incorporating an effective amount of monohydric or polyhydric polyoxyalkylene alcohols or mixtures thereof in place of water into the reaction mixture after completion of phosgenation. Although this process enables the iodine color number of the raw MDI to be considerably reduced, for example to values of less than 60, preferably from 35 to 20, this method too has dis- advantages. A disadvantage is, for example, that the addition of the polyoxyalkylene alcohols reduces the isocyanate content of the raw MDI and the chlorine content, particularly the total chlorine content, rises and the viscosity can increase with increasing storage time.